Fig. 5.

Proposed transition from magnesium calcite (Calcarea) to opal (Silicospongea) spicules based on the bilayered structure exhibited by Eiffelia. (A) Calcarean spicule with outer sheath of collagen fibrils. (B) Secondary precipitation of opal onto the collagenous sheath (as represented by Eiffelia).(C) Further increase in opal precipitation, accompanied by reduction of the calcaean sheath to an axial filament and loss of hexaradial symmetry. Mg-Ca, magnesium calcite; ACC, amorphous calcium carbonate; CFS, collagen fibril sheath; Op, opal; Sl/Um, silicalemma/unit membrane or second collagen fibril sheath.